Since the gears are not split, have you considered knurling the axles to increase the diameter slightly? All you need is a pair of toothed pliers, the finer the tooth the better. Squeeze the axles where the gears sit firmly enough to get the teeth to bite into the surface slightly. That will cause some of the metal to be pinched upward which will increase the effective diameter of the shaft and give the gears something to grip. Don't overdo it or you will risk splitting the gears.
Doing it with pliers is a bit crude but it works. If you happen to know anyone with a hobby metal lathe they might be able to do a neater job.
Dave
I'm just a dude with a bad back having a lot of fun with model trains, and finally building a layout!
.